Obtaining magnesia from magnesiacontaining rock



Patented Jan. 8, 1952 OBTAINING MAGNESIA FROM MAGNESIA- CONTAINING'ROCKRobert A; Schoenlaub, Tifiin, Ohio, assignor to a Basic Refractories,Inc., Cleveland, Ohio, a corporation of Ohio v Application January 29,1945,.Serial No. 575,088

, In my patent applications Ser. Nos. 476,496 and 486,215, which haveissued as Patents 2,414,980 and 2,433,297, respectively, I have setforth the production and separation of magnesium and calcium carbonatecrystals. One may use batch carbonation of a dolomite slurry orcontinuous carbonation in a single magnesium carbonator or continuouscarbonization in aseries of carbonators in which the degree ofcarbonation in each is controlled by the rate of supply of carbon Idioxide. Batch carbonation however, is rather expensive and variable inproduct, and continuous carbonation in a single magnesia carbonatoryields mixtures of small, medium and large crystals, which'are notparticularly easy to separate. And continuous carbonation in'a series'ofcarbonatorsis better, but retention of the crystals in a partiallyhydrated state renders them less effective for froth flotation. Thepresent invention eliminates such difficulties and makes possible thecheap preparation of'lansfordite and calcium carbonate from dolomite sothat they can be easily separated. Other objects and advantages willappear from the following description.

To the accomplishment of the foregoing and related ends, the invention,then, comprises the features hereinafter fully described, andparticularly pointed out in the claims, the following description andthe annexed drawing setting forth in detail certain illustrativeembodiments of the invention, these being indicative however, of but afew of the various ways in which the principle of the invention may beemployed.

In said annexed drawing:

The sole figure is a diagrammatic View of an apparatus arrangement inconformance with the invention. I

The raw material may be dolomite or other magnesia-providing carbonateor oxide or hydroxide rock, each as relatively pure dolomite, magnesiumlime stone, predazzite, pencatite, brucite, magnesite, etc, also thereaction product of magnesium silicate and CaO, and for conciseness allsuch are hereinafter referred to as dolomite.

The dolomite rock may be burned ina furnace or kiln, as kiln 2 in thedrawing, at any 0011- venient temperature, subject to two general limitations. First, the stone must be more or less completely calcined, andsecond, it must not be burned so hard that it will not react with waterand CO2. This is equivalent to a range of firing temperature of fromabout 1700" F. to 2600 F.

Normally, a low temperature calcination to an ignitionloss of about 1per cent is preferred.

9 Claims. (CI. 2367) Satisfactory calcine may be produced in multiplehearth roasters, rotary kilns, stack kilns, etc., provided the stone issuitable and adapted to the furnace and a proper burning schedule isfollowed.

Any convenient feeder and hydrator 3 may be used which will form auniform slurry. In some instances pulverization may be necessary butusually hydration will produce the desired comminution. If so, it ishelpful to classify out any contained coarse mattenhard core and thelike. The

slurry should not be so thick that it is difificult to handle, norso'thin that an unnecessarily large volume has to be processed.Normally, 8 per cent of calcine in 92 per cent water slurry is about amaximum density, and 6 to 7 per cent of calcine an optimum.

From the hydrator 3 the slurry is transferred to the calcium hydroxidecarbonator 4. This carbonator may be of any design or type provided itgives intimate contact between gas and slurry and sufficient agitationandhas a means of introducing fresh and removing spent flue gas. Gas andslurry are reacted in this carbonator until the pH is reduced to about10.5. This insures substantially complete carbonation of the Ca(OI-I)2.

A pH controller'can actuate a motorized valve on the gas supply to holdthe slurry closely on this control point. The slurry temperatures inthis operation should be F. or higher. Normally, due to the heat ofreaction and absence of cooling, it will be about F. The carbonatorshould be large enough that it will give an average retention time ofabout one and onehalf hours.

From the calcium hydroxide carbonator the partially carbonated slurrypasses through a heat exchanger 5 and then to a series of magneticcarbonators 6. The slurry in these carbonators should be maintained atsubstantially complete carbonation, pH about 8.5. In the first of thesecarbonators magnesium carbonate crystals are formed. Then, asthe'magnesium carbonate crystals pass on through the series ofcarbonators receiving carbon dioxide, they are continuously grown witheffective control of their development rate by the introduction ofadditional magnesiacontaining slurry in which the original calciumat'such rates as to controllably and selectively,

develop the magnesium carbonate crystals to relatively large size, andthe control is exercised by controlled additional slurry feed ratherthan by control of the CO2 feed. Its advantages will beenoughcarbonators to provide thenecessary uniformity of retention time andthey sho'uld1oe large enough to provide a sufficient average re;-

tention time. Generally, four gcarbonators-giving 7 a ratio of 5:3.between totalvolume and feed are satisfactory if the subsequentseparation is done by froth flotation, but a greater number and alargerratio of volume to feed may te ns-(seesaw I for general mechanicalseparation. Theaction of these carbonators will be discussed in more etaThe ;gas system includes the kiln o r furnace from which the gas isderived, a scrubbingtnwer 8 and blower 9 and pipes and manifolds. An

excess of gas over that required in the process is normally provided bythe combustion of the fuel andicalcination of the stone. If desired,partpfthisexcess may bewasted or disposed of elsewhere. If absorptuionis ineflicient and a deficiency of gas is encountered, this too can beremedied bypassing the gas successivelylthroiigh two or more carbonatorssothat a greater percentage oftheCOz is utilized. Any cool scrubbed fluegas containing from 10 to 40 per cent CO2 may pe used for carbonation,the lower limit bei:

ing fixedby the additional cost of handlinglarger volumes of gas, andthe upper limit by the increasing difiiculty of controlling the pH inthe magnesia carbonators. V a

Control over this carbcnization is necessary. 4

The concentration of the gas has an important bearing on the pH of theslurry and the amount of magnesia-dissolved as magnesium bicarbonatewhich is not susqptible to removal bythe usual means. tion of magnesia.carbonation with; an average CO2 content of about 19 willgivesatisfactory conditions. This concentration can be obtained byslowing'the feedof higher CO2 gasuntilthe mean CO2 content of theentering and outgoing gas is about 10 per cent. lThis control may beeither automatic or manual. The magnesia inadvertently taken intosolution may be recovered byfrecycling the effluent solution fromseparation back to the hydrator. a

Cold water for cooling may be from ponds, spray towers, wells orbyrecirculation through a refrigerator system. -I- Ieat exchange betweenslurry and water maybe accomplished by any conventional devicehavingadequate transfer surface. Economy of water can be secured by usingthe'partially heated water from the magnesia. cells for gas scrubbing,"co'oling the lime slurry, or for any condensers that may be incidentalto the process. Cooling is necessary to prevent the inversion'oflansfordite, MgCOaSHzO, to "riesquehonite, MgCOaBHzQ. Nesciuehonitei'snot as easy toseparate and will'contaminate the lime product. Theinversion temperatures are variable with conditions, but in aplan'tshould A high 002 gas will give excessive solu-Z is of animatorsiiicfasd sterner fever Lansfordite can be made to grow at a rate ofabout 10 microns per hour. Magnesia introduced to provide the substancefor growth requires an hour or two for reaction but normally this can beneglected and attention directed entirely to the growth of thelansfordite.

The first step of the .magnesium carbonate crystallization is thecarbonation of slurry in the first magnesia carbonator to form newcrystals.

10 Under conditions existing in this carbonator new 7 crystals-winautomatically form as they are needed to 'absorb the constant additionof magnesia substance and the older crystals will grow at itheirma'i'dnium rate. The number of crystals '15 fed-to the remainingcarbonators can be controlled by varying the rate of feed to thiscarbon'ato'r. increase of feed rate will increase the; number and adecrease will decrease the number, but the relation is not direct orsimple.

20 I, The succeeding carbdnators augment the size fof'the'formedc'rys'tal'sas'tli'ypa'ss through them.

Slur yand'g'a's are provided for this l urpb's'e. It

"is desirable fort'wo reasons that the growth rate in these'carbonatorsbe substantially less theme.

maximum. Firstfmaximumgrowth'rates occur capturin the rcrmauon'tf newcrystals, and a rmrginbrsafetyYMa be providedto prevent this. Second,"crystals 'grown'at their maximum rates have'aboutftwice the'oc'cludedimpurity of crystal'sgrown at ress than'their maximum 'g'rc'iiir'th"rates. A growth iat''atabout '7 microns per'hoiir is satisfactory. g

The adjustmenf'of feed. to cause desirable growth rates is complicatedby 'two'antagonis'ti'c factors. Fir'st,in passing from the first to lastagitator the number of crystals per 'unit volume availableforj'absorbing the magnesia of the'feed decreases bythe' continuousslurry addition. second, 'thear'eaiof thesecrystals'which absorbs themagnesia increases as the square of their dimension. The second factoroutweighs the first and usually enough c'rystalare'a willbe availablebut the'cjonditions "are so complex the feeds must be determinedempirically for best results.

.45, The 'numb'er'fijf carbonators in the series has an importantbearing onfthe uniformity of the crystals produced. In asing'lecarbonator there would be 'crystal'sjof all sizes representing thevarious ages offbrystals'in'suc'ha'carbonator. As

5 the "number of 'c'arbonators increases, the uniformity of crystal sizeir'np'roves since the newly formed crystals can no longer short circuitthrough the carbon'a'ting system to the same extent. Practically, acompromise is necessary,and

P5 four agitatorsar'e usually enough when the sepa- ="-i the process,the feed to the first carbonator should be increased. If,having-adjusted. therespective feeds, the. crystals of 'the productafestill too "small, the feed ito'the firstcarbonator shouldbe 1 reduced"and "either thefnumber iz'e tion of new crystals occurs in the latterpartof crystals are formed and they are grown for long- {of these aresodium palmitate, the collectors sold in the trade under the nameelastoilf'naph- 'thenic acid, sodium naphthenate, the commer-' .cialcollector known as tetranol," the commercial distillate from pine tarknown as pineol 345-A" sodium oleate, etc. For example, sodiumnaphthenate in amounts of 1.2 pounds per ton of solids may be employed.With such collectors, the magnesium carbonate crystals are collected inthe froth, and the calcium carbonate goes in the tailings. The detail offlotation may be as in my application Serial No. 486,215.

Generally, it is preferred to operate the carbonation of the magnesiumhydrate to the formation of lansfordite, MgCO3.5H2O; but where desired,the conditions may be operated to the production of nesquehonite,MgCOa.3H2O. The latter involves a higher temperature adjustment, as forinstance 70 F. or more.

As an example: I calcined dolomite of the Niagara Formation fromNorthern Ohio of the following approximate composition:

This was calcined in a size of about mesh in a multiple hearth furnace,to an ignition loss of about 1%, and formed into a slurry in a ratio ofabout 7 parts of calcine to 93 parts of water. The slurry was thencarbonated to a control point of pH 10.5 in a continuous carbonator ofabout 100 gallons capacity and at a temperature substantially above 100F. I pumped 80 gallons per hour divided into 4 more or less equal partsto each of 4 identical carbonators. These carbonators have a capacity of100 gallons and are so arranged that the crystals forming in one,progress through the others in the manner described. I continuouslycarbonated at a temperature of about 57 F. with scrubbed flue gas ofabout per cent CO2. I obtained a slurry product comprising mixtures offine calcium carbonate, of about 1-2 microns in size and lansforditecrystals between about 50 and 70 microns in size with a few crystalssmaller or larger than these limits.

These compounds when separated by flotation, classification, screeningor otherwise have approximately the following composition expressed on acalcined basis:

. 6 Other'mods of applying the principle of the invention maybeemployed, change being made as regards the details described, providedthe features stated in any of the following claims, or the equivalent ofsuch, be employed.

I therefore particularly point out and distinctly claim as myinvention: 1. A process of the character described, whichcomprises-supplying carbon dioxide to a slurry of calcined dolomite to apH of about 10.5, at a temperature substantially above F., therebyforming fine calcium carbonate crystals, then at a temperature nothigher than about-57 F. supplying additional carbon dioxide to thepartially carbonated slurry and adding fresh partially carbonated slurryin controlled amount from the first carbonation for reaction therewithat such "rates as to selectively form lansforditecrystals of relativelylarge size, and separating the magnesium carbonate and the calciumcarbonate without destroying the crystal form.

2. A process of the character described, which comprises supplyingcarbon dioxide to a slurry of calcined dolomite to a pH of about 10.5,thereby forming calcium carbonate crystals, then at a temperature below60 F. supplying additional carbon dioxide to the partially carbonatedslurry and adding fresh partially carbonated slurry in controlled amountfrom the first carbonation for reaction therewith at such rates as toselectively form lansfordite crystals of relatively large size, andseparating the magnesium carbonate and the calcium carbonate.

3. A process of the character described, which comprises supplyingcarbon dioxide to a slurry of calcined dolomite to a pH of about 10.5,at a temperature substantially above 100 F., thereby forming calciumcarbonate crystals, then at a temperature below 60 F. supplyingadditional carbon dioxide to the partially carbonated slurry and addingfresh partially carbonated slurry in controlled amount from the firstcarbonation for reaction therewith at such rates as to selectively formmagnesium carbonate crystals, and separating the crystals.

4. A process of the character described, which comprises supplyingcarbon dioxide to a slurry of calcined dolomite to a pH of about 10.5,thereby forming calcium carbonate crystals, then at a lower temperaturesupplying additional carbon dioxide to the partially carbonated slurryand adding fresh slurry from the first carbonation for reactiontherewith at such rates as to selectively form magnesium carbonatecrystals of relatively large size, and separating the crystals.

5. A process of the character described, which comprises supplyingcarbon dioxide to a slurry of calcined dolomite thereby forming calciumcarbonate crystals, then at a lower temperature supplying additionalcarbon dioxide to the partially carbonated slurry and feeding additionalslurry from the first carbonation for reaction therewith at such ratesas to selectively form lansfordite crystals of relatively large size,and separating the crystals.

6. A process of the character described, which comprises supplyingcarbon dioxide to a slurry of calcined dolomite thereby forming calciumcarbonate crystals, and then subjecting the slurry to carbon dioxide andadditions of fresh slurry from the first carbonation for reactiontherewith at such rates as to selectively form lansfordite crystals.

7. A process of the character described, which comprises supplyingcarbon dioxide to a slurry awe-71s Eofcalcined *dolcmite thereby formingcalcium carbonate, and then producing magnesium carbonate crystals inthe slurry by supplying additional carbon dioxide. and-feeding freshslurry from the first carbonation for reaction therewith at such ratesas toselectivelyfavor magnesium carbonate crystal growth." r I 8: Aprocess of the character described, which comprises supplying carbondioxide to a slurry of calcined dolomite thereby forming calciumcarbonate, and then selectively producing lansfordite crystals in theslurry by additional carbon dioxide and feeding .fresh slurry from thefirst carbonation. 7

9 A processof the character described, which :comprises'. supplyingcarbon dioxide to a slurry of calcine'd dolomite thereby iorming'calcium:carbonate, and then .selectiyel'y producing magmums nesium carbonate inthe slurry by additional carbon. dioxide and feeding 'fresh slurry:Erom' the first carbonation.

ROBERT AJSCHQENLAUB.

REFERENCES CITED .7

The following references are of record in the file of this patent:

I UNITED STATES PATENTS

1. A PROCESS OF THE CHARACTER. DESCRIBED, WHICH COMPRISES SUPPLYINGCARBON DIOXIDE T A SLURRY OF CALINED DOLOMITE TO A PH OF ABOUT 10.5, ATA TEMPERATURE PERATURE SUBSTANTIALLY ABOVE 100* F., THEREBY FORMING FINECALCIUM CARBONATE CRYSTALS, THEN AT A TEMPERATURE NOT HIGHER THAN ABOUT57* F. SUPPLYING ADDITIONAL CARBOPN DIOXIDE TO THE PARTIALLY CARBONATEDSLURRY AND ADDING FRESH PARTIALLY CARBONATED SLURRY IN CONTROLLED AMOUNTFROM THE FIRST CARBONATION FOR REACTION THEREWITH AT SUCH RATES AS TOSELECTIVELY FORM LANSFORDITE CRYSTALS OF RELATIVELY LARGE SIZE ANDSEPARATING THE MAG-